1. Field of the Invention
The invention relates to a process for gaining or enrichment of fructose and, more particularly, to the fermentation of glucose accompanying fructose by a mutant of Zymomonas mobilis which does not decompose fructose.
The invention also relates to Zymomonas mobilis mutants which can be used for the purpose of producing fructose from glucose/fructose mixtures.
2. Description of the Prior Art
Because fructose is one and one-half times as sweet as sucrose, it is a valuable sweetener, which is used primarily for pharmaceutical purposes and for low-calorie foods. Glucose and fructose are both sugars which are included in the family of compounds comprising carbohydrates of which starches also form a part.
The customary processes for the extraction of fructose start with invert sugar or High Fructose Corn Syrup (HFCS), that is, mixtures of glucose and fructose. From this, the fructose is derived by separation processes such as crystallization, precipitation or chromatography, and sometimes with complexing, for example, with boric acid or conversion of the glucose into gluconic acid. These separation processes are relatively expensive because of the similarity of the two sugars and, therefore, attempts have already been made to develop enzymatic or microbial processes.
U.S. Pat. No. 4,356,262, which is incorporated herein by reference, describes a process for obtaining fructose syrup and ethanol from a sucrose solution. In this process, fructose polymers and ethanol are formed by Pullalaria pullulans and yeasts such as Saccharomyces bailii or cerevisiae by means of fructosyltransferase, and can then be hydrolyzed into fructose. But this process, which begins with sucrose, requires numerous complicated preparation and reaction steps. To get the special enzyme necessary for the transfructolysis, a special micro-organism must be cultivated in advance and prepared. The fermentation yeast, which must be added at the same time as the enzyme or added in a separate step, then converts the resulting glucose directly into ethanol. The technically very desirable fructose is obtained only as a polymer, and must subsequently be hydrolyzed in an additional step and under difficult conditions. Technically, such a complicated process seems far too expensive for obtaining fructose.
In Australian Patent Abstract No. AU-A 29 530/84, which is incorporated herein by reference, a process is described for the production of fructose and ethanol from sucrose, where the sucrose concentration must be greater than or equal to 30 percent by weight. By means of the bacterium Zymomonas mobilis, preferably after pre-treatment of the sucrose solution with an immobilized levan sucrase enzyme preparation (EC 2.4.1.10), there is a selective conversion of the glucose to ethanol within 1-2 days. However, this process also has significant disadvantages, in that the enzyme levan sucrase, according to EC 2.4.1.10, is in effect a fructosyltransferase, that is, the reaction products from sucrose are polyfructose (levan) and glucose. (T. E. Barman, 1969, Enzyme Handbook I, page 310, which is incorporated herein by reference.) It is also known that Zymomonas also produces levan from sucrose (D. W. Ribbons, 1962, Biochem. J. 82, 45 p; E. A. Dawes et al., 1966, Biochem. J. 98, 804; K. J. Lee, 1981, Biotechnol. Lett. 3 207, incorporated herein by reference.) The process according to the Australian Patent AU-A 29 530/84 also leads to the formation of levan, and therefore to significant slime formation. Naturally, the formation of levan also causes losses of monomer fructose. In addition, known strains of Zymomonas mobilis also convert fructose to ethanol, which also necessarily decreases the fructose yield in the above process. (Swings and DeLey, 1977, Bacteriol. Rev. 41, 1, incorporated herein by reference.)
A presentation was made at the 11th International Carbohydrate Symposium in Vancouver (S. Bringer, M. Scollar, H. Sahm, 1982, Abstract No. 39, incorporated herein by reference) concerning tests for the derivation of fructose-negative mutants of Zymomonas mobilis. The mutants indicated for this purpose, however, turned out to be unsuitable for practical use in the enrichment of fructose from mixtures of fructose and glucose, since the mutants were too unstable and unexpectedly lost their selectivity again even after a short time.
According to the known and sometimes very expensive methods, therefore, large proportions of undesirable by-products are produced, or the initially suitable microorganisms are stable only for a short time.